In the development of magneto-optic storage media for optical memory device applications, recent interest has been focused on thin films of amorphous rare earth-transition metals in binary alloy such as TbFe or GdFe and ternary alloys such as TbGdFe or TbCoFe or equivalent alloy systems, as suitable magneto-optic materials for the storage media because of the unusual anisotropy and magneto-optic properties of such metals. For information storage applications based on thermo-magnetic writing, the bit information is stored at discrete areas by writing with a focused high power laser in combination with an applied magnetic field. To read or retrieve the stored information, the bit areas are scanned with a lower power, focused, polarized laser beam, in combination with photodetector and analyzer, resulting in a read back signal.
For practical applications, the written bit areas should be kept as small as possible to maximize the storage capacity of a given portion of medium. However, decreasing the bit area size also decreases the read back signal amplitude level and hence reduces the signal-to-noise ratio of the medium. Several structures, such as overcoating the medium with a high refractive index dielectric layer of appropriate thickness or providing a reflective underlayer, previously have been proposed to enhance the read back signal level from the medium during information retrieval by causing the read back signal to experience increasing mode conversion (polarization rotation). As explained hereinafter, the previously proposed structures (dielectric overcoat or reflective undercoat) do provide mode conversion of the read beam incident on a bit area but with a limit. Accordingly, there remains a need for a new medium structure that will provide further increase in mode conversion of the read beam incident on a bit area over the previously proposed structures, which will result in a significant increase in signal-to-noise ratio and hence permit a significant increase in bit density.